Quite often the equipment, mechanisms, processes, and vehicles (earthbound or for use in space) required by our complex society dictate the necessity for maintaining a positional alignment between the equipment, or a mechanism thereof, and a given reference. The given reference may well be the true horizontal; and the positional alignment is usually one of angular position of the equipment, or a part of its mechanism, with respect to the true horizontal.
Equipment and mechanisms for sensing the angular position of a body with respect to the true horizontal are readily available. Many of such provide an electrical signal, indicative of an angular displacement of a body with respect to the horizontal, for subsequent use by servo-type mechanisms to take corrective action that realigns the body to the true horizontal if desired. Some available position sensors utilize a pendulum in conjunction with other components to sense and indicate angular position, such as that shown and described in U.S. Pat. No. 4,163,325 granted on Aug. 7, 1979 to D. Hughes for Verticality Sensors. Other position sensors utilize a source of radiation and associated optical elements, such as that shown and described in U.S. Pat. No. 4,159,422 granted on June 26, 1979 to S. Okubo for Temperature Stable Displacement Sensor With Fine Resolution. However, such position sensors are cumbersome and bulky and could prove to be unusable in environments where weight, space, response time and similar criteria are essential or critical.
Some available position or tilt sensors utilize a glass vial or tubular container within which an air or gas bubble is disposed in an electrolytic solution and to which suitable electrodes are attached. As the position of the tube in such liquid level devices moves with respect to the horizontal so does the relative position of the bubble and electrolyte with respect to the electrodes. A current passed between electrodes and through the electrolyte provides an indication of the angular position of the container. However, devices such as those shown in: U.S. Pat. No. 2,977,559 granted on Mar. 28, 1961 to A. M. Rosenberg et al for Low Resistance Electrolytic Tilt Device; in U.S. Pat. No. 3,114,209 granted on Dec. 17, 1963 to F. B. Foody et al. for Level Sensor; and in U.S. Pat. No. 3,299,523 granted on Jan. 24, 1967 to L. N. Lea for Levels fail to effectively isolate the electrolyte in the container from the effects ambient temperature and its changes.
It is the change in electrical resistance of the fluid within the container of these liquid levels that is sensed and utilized as the basic indication of change in angular position for these devices. However, the electrical resistance of the fluids utilized for such sensors is also dependent upon the temperature of the fluid. Thus, changes in resistance of the fluid due to changes in temperature will provide a false indication of position if not accounted for. Correcting for such unwanted resistance changes can be cumbersome, expensive, time consuming and render sensors subject to temperature changes unusable for applications where thermal isolation and stability are required.
Still other available sensors seek to provide some degree of isolation from changes in ambient temperature by utilizing insulating blocks for the liquid container as shown and described in U.S. Pat. No. 2,713,727 granted to L. L. Balsam on July 26, 1955 for Linear Bubble Level Signal Device; or by placing the liquid container in a container within a container as shown and described in U. S. Pat. No. 4,312,131 granted on Jan. 26, 1982 to P. J. Scriffignans et al. for Accurate Level Sensor. But, such sensors also do not provide static and dynamic thermal enviornments for the fluid wherein the fluid temperature remains isothermal (i.e. no temperature gradients within the level).
Available tilt sensors do not readily satisfy the general stability requirements for sensors utilized in devices such as the Sperry Tilt Meter, or in similar applications. Those general stability requirements dictate that the reference or null position should be stable to better than 1/4 of an arc minute through temperature excursions of + and -50.degree. F. from normal room temperature. This requirement applies to steady state conditions. A rate of change of temperature also applies. The same allowable change must not be exceeded with temperature changing at the rate of 100 degrees/hour.